1. Field
This disclosure relates generally to charge pumps, and more specifically, to charge pumps having ramp rate control.
2. Related Art
Charge pumps are commonly required for use with non-volatile memories (NVMs) because voltages required for effective program and erase are greater than the power supply voltages. Some NVM technologies require a charge pump voltage with a controlled ramp rate. Typically the ramp rate is either controlled by a shunt current Ish that consumes some of the charge produced by the pump as shown in FIG. 1. As an alternative the size of the charging capacitor is adjusted to modulate the amount of charge produced by the pump as shown in FIG. 2. In each case the ramp rate is proportional to the size of a smoothing capacitor (Csm) and a load capacitor Cl, and the magnitude of the load Ild and shunt currents Ish. The ramp rate is inversely proportional to the charging capacitor (Cp) and the clock frequency.
Selecting a shunt current or a charging capacitor size can be complicated by the variability of the load capacitance and current and overall power consumption. Those values will depend on the number of bits that are programmed; precise control of the ramp rate for all operating conditions using prior art is therefore impossible. The situation can be mitigated somewhat by selecting a large smoothing capacitor that swamps the effect of changing load capacitance and current at the expense of die area and increased power consumption during ramping. Without the increase in smoothing capacitor an increase in load can dramatically slow the ramp rate as illustrated in the simulation shown in FIG. 3 that compares two loads, one of which is 2.5 times the other. For the simulation shown in FIG. 3 the pump consisted of a single stage with 3V input and 5V regulation point.
Thus there is a need for a charge pump that improves upon one or more of issues raised above.